The field of the present invention relates, in general, to Shape Memory Alloy (SMA) actuators and elements comprising these alloys. More specifically, the field of the invention relates to a spatially distributed activation means for controllably altering the local shape and deflection forces of a SMA sheet.
Materials which change their shape in response to external physical parameters are known and appreciated in many areas of technology. The geometry of a piezoelectric crystal, for example, is altered by an electric field. Similarly, the macroscopic shape of a SMA is sensitive to temperature. A SMA material undergoes a micro-structural transformation from a martensitic phase at a low temperature to an austenitic phase at a high temperature. When in the martensitic or low temperature phase, a SMA exhibits low stiffness and may be readily deformed up to 8% total strain in any direction without adversely affecting its memory properties. Upon being heated to its activation temperature, the SMA becomes two to three times stiffer as it approaches its austenitic state. In addition, at the higher temperature, the SMA attempts to reorganize itself on the atomic level to accommodate a previously imprinted or "memorized" shape. Useful motions and forces may be extracted from a SMA element as it attempts to move to its previously memorized shape. If permitted to cool, the SMA returns to its soft martensitic state.
A shape may be "trained" into a SMA by heating it well beyond its activation temperature to its annealing temperature and holding it there for a period of time. For a TiNi SMA system, the annealing program consists of geometrically constraining the specimen, and heating it to approximately 510 C for fifteen minutes. In most cases, functionality is enhanced by leaving in a certain amount of cold working by abbreviating the anneal cycle.
The point at which a SMA becomes activated is an intrinsic property of the material and is dependent on stochiometric composition. For a typical shape memory alloy such as TiNi (49:51), a change in alloy ratios of 1% produces a 200 C shift in transition temperature. Binary SMAs such as TiNi (sometimes referred to as Nitinol) can have a large range of transition temperatures. For Nitinol, atomic composition can be adjusted for a phase transition as high as 100 C and as low as -20 C or more. Sub-zero transition materials exhibit superelastic behavior. That is, they can reversibly endure very large strains at room temperature. In the medical community, superelastic formulations of Nitinol are commonly employed in "steerable" guidewires.
In contrast to the passive characteristics of a superelastic SMA, an actuator that must perform work on its environment requires a SMA capable of producing useful forces and motions for a given input of thermal energy. Because most thermal devices must expel their waste heat to the ambient environment, which in most cases is near room temperature, higher transition point SMAs are most commonly used as active actuator elements. During phase changes, a SMA will exhibit a maximum recoverable strain of up to 8% while producing a recovery force of 35 tons per square inch or more.
It is known to use SMA actuators in conventionally steerable elements such as catheters. One such application, as described in U.S. Pat. No. 4,543,090, involves a conventional steerable and aimable catheter using SMAs as the control elements. This device and other conventional steerable devices using SMA elements are severely limited in dexterity. Movement is limited to a single plane.
Upon cooling, a SMA element does not necessarily return to its pre-activation shape. Thus, to attain reversible motion, a means must be provided to return the inactive SMA element to a shape other than its trained shape. This can be accomplished with active or passive components. In the passive configuration, a return spring is provided such that it is just strong enough to fully deflect the SMA element in its martensitic state. When activated, the SMA element possesses enough force to overcome the return spring and perform work on the environment as it approaches its memorized state.
In an active or antagonistic configuration, each SMA element must be coupled to at least one other SMA element. When one SMA element has been heated to an activation threshold, it provides sufficient force to deflect the inactive actuator in a desired direction. Reverse motion is accomplished by reversing the order of activation.
A contraction-extension mechanism using joints made of an SMA material is shown by Komatsu et al. in U.S. Pat. No. 5,335,498. The described mechanism is an actuator strip with multiple joints. Joule heating elements or shape-controlling heaters are integrally attached to the component joints of the actuator. Passing sufficient current through the heaters causes the strip to contract at the joints in a bellows-like fashion. Three-dimensional motion can be imparted to objects by a geometrically suitable arrangement of such actuators. Unfortunately, the extension-contraction mechanism is also limited. Each strip contracts and extends in one direction only. Conventional arrangements of SMA strips to impart three-dimensional motion to objects are impractical because such structures are unduly large and cumbersome. This is due to the fact that such structures are not locally controllable and require excessive amounts of energy for their operation.
U.S. Pat. No. 5,405,337 issued to the present applicant teaches a flexible VLSI film containing SMA actuator elements and associated control and driver circuitry. The film is wrapped around any bendable element, such as a flexible, hollow tube, catheter, or the like. Thus, the SMA actuator elements are spatially distributed about the circumference of a bendable element. In one aspect of the invention, a distributed SMA array is provided on a flexible insulating film by sputtering a SMA alloy and patterning the individual islands of material with reactive ion etching (REI), plasma assisted etching, liftoff, or the like. The individual SMA actuators can then be directly heated with electrical current (conductive SMA), or may be heated by contact with an adjacent heat source (non-conductive SMA). Since the SMA actuator film is wrapped around a flexible tube, activation of the SMA film achieves movement in three dimensions.
Although this approach is effective, the associated manufacturing costs are high. Patterning the SMA film using conventional VLSI methods can be expensive and sputtered SMA films thicker than approximately 10 microns are difficult to produce at the present time. The stress accumulated within a sputtered film greater than this thickness usually causes the film to rupture. However, current efforts involving heated substrate sputtering may mitigate these damaging internal stresses.
A second problem with sputtered SMA materials is that the atomic composition and form of the sputtered film may differ significantly from that of the parent target. For example, in the case of a binary SMA such as 50/50 TiNi, when the sputtering ions strike the surface of a target and liberate individual atoms of Ti and Ni, the difference in vapor pressure between these two elements produces a significant change in the 50/50 composition in the vapor phase and subsequent deposition phase. In addition, the grain structure of the deposited film must be carefully controlled for efficient SMA actuation.
What is needed then, is a low cost method for producing a distributed SMA actuator array which does not rely heavily on VLSI patterning and sputtering techniques. In particular, it would be advantageous to obtain a sheet of SMA material directly from bulk, wire or plate stock without adversely altering grain structure or composition. A distributed array of addressable heaters and associated electronics could then be patterned directly on the SMA film. It would also be beneficial to limit the number of cuts made in the SMA film such that an automated saw, abrasive water jet, laser cutter, electronic discharge machining, or the like, could be employed to an economic advantage.